1. Field of the Invention
The present invention relates generally to an electrochemical cell and more particularly, to a new and improved electrochemical cell that is adapted for in situ scattering studies of the interface between a working electrode of the cell and electrolyte contained in the cell based on transmission geometry.
2. Background of the Invention
In an electrochemical cell, a solid working electrode is submerged in a liquid electrolyte. X ray techniques have been increasingly used in order to investigate in situ the interface between the working electrode and the liquid electrolyte. For the most part, extended X-ray absorption fine structure (EXAFS) techniques have been used to probe for chemical or short-range structural information as to the interface between the electrode and the liquid electrolyte. Typically, the EXAFS measurements are accomplished in a reflection geometry.
In such a reflection geometry, a membrane has to be pulled taut over the working electrode surface so that the solution layer through which the X rays must travel is minimized. An incoming X-ray beam is transmitted through the membrane and the electrolyte onto the working electrode. A detector receives the reflected beam and determinations can be made about the surface from which the X-ray is reflected by scattering techniques. When such a reflection geometry is used, the X-rays are reflected not only from the electrochemical interface between the working electrode and the electrolyte, but also from the air/membrane and membrane/solution interfaces through which the incoming X-ray beams must pass. As a result, the reflective data obtained when a reflection geometry is used requires deconvolution or subtraction of the interference that occurs from the air/membrane and membrane/solution interfaces. In addition, the nature of those additional interfaces also may vary making it even more difficult to correctly interpret the data received.
Other factors also tend to make it more difficult to correctly interpret data obtained when using reflection geometry. The current distribution over the working electrode surface in an electrochemical cell may be nonuniform due to the geometric arrangement of the electrodes and the large ohmic resistance in the electrolyte gap. The electrolyte gap may be expanded by inflating the membrane or withdrawing the electrode assembly during electrochemical manipulations. Yet, the electrolyte gap is constricted during the X-ray measurements so that meaningful data may not be obtained. In addition, absorption corrections must be made when using reflection geometry due to the change at small angles in the path length over which X ray beams travel. Moreover, there is a tendency for the diffuse background to rise rapidly at small scattering angles in the reflection geometry. Therefore, the total signal is reduced by absorption but the diffuse background from the air/membrane and the membrane/solution interfaces are hardly attenuated resulting in a low signal to background ratio at small angles. As a result, it would be preferable to use transmission geometry when analyzing the interface between an electrode and electrolyte in an electrochemical cell. In the case of a transmission geometry, the X-ray beam could be transmitted through membranes disposed transversely to and on opposite sides of the electrode. As a result, the X-rays reflect to the detector only from the electrochemical interface and the reflectivity data can be interpreted directly instead of requiring the processing needed in the case of reflection geometry. Moreover, little absorption correction is required in using transmission geometry because the path length of the X-rays is nearly constant over the angular range of interest. In addition, the diffuse background is attenuated as much as the signal in the transmission geometry such that the signal to background ratio remains constant over the full angular range of interest.
Consequently, it would be advantageous to utilize transmission geometry instead of reflection geometry in studying the electrode/electrolyte interface in an electrochemical cell. However, a suitable electrochemical cell has not heretofore been available that is constructed such that transmission geometry can be readily utilized.